Stabilized imaging system

ABSTRACT

A stabilized imaging system in which the lens assembly is fixed, and an electro-optic imager element is moveable to compensate for three-dimensional movements of the surrounding structure. Preferably the optical train also includes a movable prism, which can rotate the field of view in one plane. Rotation of the imager compensates for the image rotation caused by rotation of the prism.

BACKGROUND OF THE INVENTION

Effects of Platform Motion

Electro-Optic Reconnaissance

Image Rotation

SUMMARY OF THE INVENTION

BRIEF DESCRIPTION OF THE DRAWING

DESCRIPTION OF THE PREFERRED EMBODIMENTS

General System Configuration

Lens Assembly

Imager

Mechanical Implementation

Control

Signal Processing

Prescaling

Analog filtering

Image Brightness Compensation

Gain Control

Adaptive Time Constant

Image Rectification

Bandwidth Limiting

Data Output

ABSTRACT

BACKGROUND OF THE INVENTION

The invention relates to imaging systems. The present inventionparticularly relates to aerial reconnaissance systems and methods.

Various known teachings which are believed to be related to various onesof the innovations disclosed herein will now be discussed. However, itshould be noted that not every idea discussed in this section isnecessarily prior art. For example, characterizations of particularpatents or publications may relate them to inventive concepts in a waywhich is itself based on knowledge of some of the inventive concepts.Moreover, the following discussion attempts to fairly present varioussuggested technical alternatives (to the best of the inventor'sknowledge), even though the teachings of some of those technicalalternatives may not be "prior art" under the patent laws of the UnitedStates or of other countries.

In aerial reconnaissance, the cost of each reconnaissance run isrelatively large. It is therefore desirable to obtain a large amount ofinformation from each run.

In one class of aerial systems, the motion of the air vehicle provides ascanning motion in one direction. Thus, it has long been recognized thata continuous sequence of linear images of the ground provides aconvenient way to assemble a large-area image. In such a system, one ofthe determinants of image resolution is the lateral resolution withinthe image track.

Effects of Platform Motion

In a fixedly mounted reconnaissance system, any change in the vehicle'sattitude will cause a corresponding movement of the image on the focalplane. This movement may be very large. Thus, reconnaissance underturbulent air conditions presents some inherent difficulty in attaininghigh resolution output, since the aerodynamically-caused movement of thevehicle may degrade resolution of the image. For example, in a systemwhich is operating at 5000 feet, and which has a lateral resolution of12,000 pixels across a 45 degree field of view, a roll axis attitudechange of only one degree would cause a lateral image shift acrossseveral hundred pixels. Thus very small attitude changes have thepotential to cause drastic shifts in the image.

It has therefore been recognized in the art that compensation foraircraft attitude changes is highly desirable. One previously suggestedway to accomplish this is to use a camera (or electro-optic imager)assembly which is mounted on gimbals, and mechanically driven forstabilization. Another approach, used with film-based photographicsystems, has been to actively move the film magazine. See, for example,U.S. Pat. Nos. 3,092,687, 3,503,663, 3,638,502, and 3,982,255, which arehereby incorporated by reference. However, many such approaches have thedisadvantage that the moving mass is relatively large. This means thathigh frequency components of instability cannot be optimallycompensated. Moreover, a mechanism which is able to move the wholeassembly is likely to be relatively bulky, heavy, and expensive,particularly in view of the environmental constraints on such anassembly.

Turbulence is particularly likely to cause transient instabilities alongthe roll and pitch axes of an aircraft. Thus, one problem with priormethods is that the available quality of aerial reconnaissance has beendependent on air turbulence conditions. This is particularly a problemin military applications, where information may be needed urgently.

In reconnaissance systems for military applications, it is highlydesirable for the air vehicle in which the reconnaissance equipment ismounted to be able to take evasive maneuvers. In many previous aerialreconnaissance systems, an aircraft running a reconnaissance missionmust be very restrained in maneuvering. This makes it a better target.If the changes in the air vehicle's attitude cannot be performed withoutinterrupting the reconnaissance run, then the air vehicle's survival andits mission are inherently in conflict, which is not an ideal situation.Thus, it would be highly desirable if aircraft (or other air vehicles)could maneuver with more freedom during reconnaissance runs. This wouldalso be advantageous for non-military aerial surveying applications,since there would be less need for precise control of the aircraftcourse track during the surveying run.

One conventional type of reconnaissance system forms a line image onto amoving roll of film. Such systems inherently provide a slight degree ofresistance to roll instabilities, since to some degree the effect ofroll instabilities would show up merely as a wariness of the lines alongthe track of the airplane. However, such systems do not fully compensateeven for roll instabilities, since, depending on the exposure time ofthe film (determined by the width of the slit and the speed of filmtransport), there still may be some blurring of detail. Moreover, suchsystems are vulnerable to pitch axis instabilities. Moreover, even ifall of the information is present on the film, image interpretation maystill be difficult if the image is distorted.

In addition to roll instabilities, air vehicles will also commonly havepitch instabilities of large magnitude. Pitch instabilities due toturbulence, or changes in pitch attitude due to maneuvering, areparticularly likely to occur at a relatively high angular change rate,and therefore have a large potential to degrade imaging performance.

Yaw variations (i.e. rotations of the air vehicle around the geometricaxis which would be vertical during normal level flight) can also becaused by turbulence, although the magnitude of these instabilities willtypically be much smaller than those about the pitch and roll axes.However, yaw variations are an essential component of maneuvering anaircraft. Therefore, reconnaissance during maneuvering would beimpractical without some way to compensate for yaw attitude changes.

An airplane may also have a "crab" component, where the track vector (inthe ground frame of reference) is not perfectly aligned with theprincipal forward axis of the air vehicle. This will commonly occurwhere an airplane is flying in a cross-wind, and may also be caused byaerodynamic imbalance in a damaged aircraft. This component of motionwill appear, at the focal plane, as a fixed offset or slowly varyingcomponent of yaw attitude. At moments when the cross-wind (at theplane's location) changes rapidly, there will in fact be a transient yawcomponent.

Electro-Optic Reconnaissance

In electro-optic systems, an optical train images ground features ontoan imager, and the imager measures the image intensity at a number oflocations. (Each such location is referred to as a picture element, or"pixel.")

There are significant potential advantages to using electro-opticsensing methods in aerial reconnaissance applications. However, normalarea imaging formats are not at all suitable. For example, standard NTSCimage format is less than 600 pixels wide, but this falls far short ofthe resolution required in many aerial reconnaissance applications. Forexample, reconnaissance cameras using roll film will often haveresolutions of 20,000 equivalent pixels or more in width.

One potential advantage of electro-optic devices in reconnaissancesystems is that data can be transmitted to remote locations, withoutawaiting physical transfer of film. Another potential advantage is thatthe delays and logistics requirements of emulsion processing can beavoided. Another potential advantage is that the output of anelectro-optic imager is inherently better suited to interface to theimage-recognition algorithms which may be developed in the future.Another potential advantage is that, as the capability to make imagesmore understandable by preprocessing them advances, the output ofelectro-optic imagers will improve accordingly.

One well-known type of electro-optic imager is a charge-coupled device,or "CCD." A CCD is a semiconductor device wherein each imaging site is apotential well for minority carriers (normally electrons). Eachpotential well will collect electrons generated by photon absorption inits vicinity. The CCD output indicates the amount of charge collected ineach well, and therefore the photon flux seen at each well.

Often a linear imager will be used, so that what is imaged at eachinstant is a strip on the ground. The motion of the platform sweeps thiscoverage along the ground, at the speed of the platform, to produce alarge combined multi-strip image. In such systems, the use of an imagerwhich has a large number of pixels will help to achieve high resolutionin each strip (and therefore high cross-track resolution in the combinedimage), subject to the constraints of the optics. For example, it hasbeen suggested that a linear CCD could be used as an electro-opticsensing element in an aerial reconnaissance system. See Rachel andRoberts, "Evaluation of the Electronic Wide Angle Camera System," atpage 129 of the proceedings (designated volume 137) of the SPIEconference on Airborne Reconnaissance III (1978), which is herebyincorporated by reference. Note that this publication suggests that alinear CCD can be thought of as analogous to a scanning slit used toexpose film.

Image Rotation

Published European Patent Application No. 0-127-194 (Application No.84200649.6, filed May 8, 1984, claiming priority of French ApplicationNo. 8307911, filed May 11, 1983) shows an optical system mounted in afixed nacelle on an aircraft. Rotatable elements permit pointing thefield of view in any desired direction within a half sphere. Thisapplication recognizes that the rotation of the pointing elements willintroduce a rotation into the image. This application further teachesthat the apparent image rotation can be removed by use of a Pechan prism(shown as element P in the drawings).

SUMMARY OF THE INVENTION

Various innovative teachings will now be discussed, and some of theirrespective advantages described. Of course, not all of the discussionsin this section define necessary features of the invention (orinventions), for at least the following reasons: 1) various parts of thefollowing discussion will relate to some (but not all) classes of novelembodiments disclosed; 2) various parts of the following discussion willrelate to innovative teachings disclosed but not claimed herein; 3)various parts of the following discussion will relate specifically tothe "best mode contemplated by the inventor of carrying out hisinvention" (as expressly required by the patent laws of the UnitedStates), and will therefore discuss features which are not necessaryparts of the claimed invention; and 4) the following discussion isgenerally quite heuristic, and therefore focusses on particular pointswithout explicitly distinguishing between the features and advantages ofparticular subclasses of embodiments and those inherent in the inventiongenerally.

The electro-optic sensing system provided by the present invention has alarge capability to compensate for various aircraft movements. Thissystem can compensate not only for instabilities caused by aerodynamicforces or control logic, but also for large-magnitude attitude changescaused by maneuvering.

In compensating for aircraft movements, there are at least twoconstraints to consider: first, the compensation system, considered as awhole (including both electrical and mechanical characteristics) musthave a sufficiently large bandwidth for response that rapid changes inattitude can be adequately compensated. (This is particularly importantin compensating attitude changes in the pitch and roll axes.) Second,the magnitude of compensation possible should preferably be large. Thatis, for example, a system which could compensate for a few degrees ofroll attitude change might be useful in compensating for attitudechanges caused by random turbulence, but would not be able to compensatefor normal maneuvering. By contrast, a system (like the presentlypreferred embodiment) which can fully compensate for a roll attitude of50° from level flight will permit the aircraft to perform an extensiverange of aerodynamic maneuvering without interrupting the reconnaissancemission.

The present invention is particularly advantageous in medium-altitudereconnaissance applications. (An example of such an application would beuse in an air vehicle which had an average speed of 500 knots at analtitude of 5000 feet.) In such applications it may be necessary toimage only a relatively small field of view (e.g. 20° ), whichsimplifies some of the constraints on the optics. Moreover, therelatively large altitude means that net shifts in the aircraft positionwill have a relatively small effect on the image dimensions, if theattitude changes can be appropriately compensated. Thus, for example, arapid change in altitude of 100 feet in an aircraft at 5,000 feet,would, as a first order approximation, cause only about a two percentchange in image size. Similarly, a rapid pair of turns, which leaves theaircraft flying on a new track shifted 500 feet from its previous track,would require only about a 6° change (in the roll axis pointingcomponent of the optical train) to continue imaging essentially the sametrack on the ground. (Any resulting shift in perspective is strictly asecond order effect. For example, a lateral shift of ten percent ofaltitude would cause about one percent apparent elongation of areas atthe far edge of the field of view. Moreover, this elongation could bereduced, if necessary, by laterally shifting the imager within the focalplane.)

A further advantage of this system, as compared to systems where thewhole camera assembly is pivoted on gimbals, is that the external window112 can be smaller than would otherwise be required. That is, since thefore-aft field of view of the lens assembly 100 is predetermined, thewindow 112 need be made no larger (in the fore-aft dimension) thannecessary to accommodate this field of view. This in turn providesadvantages of cost, weight, and reliability of window 112.

Full multi-axis compensation for attitude changes not only permits thereconnaissance mission to be continued, it also simplifiesinterpretation of the data. That is, the image data provided by thepresent invention will essentially correspond to sequential measurementsalong a single straight track. It is much easier to directly interpretsuch data, since transformations to preserve geometric relationships arenot required.

It should be noted that yaw axis attitude changes, while less importantfor instability compensation than roll and pitch axis changes, are anecessary component of a complete maneuvering compensation system. Thatis, a system which can compensate for roll pitch and yaw attitudechanges can compensate for any attitude changes (within the range ofavailable compensation). Thus, for example, one advantage of the presentinvention is that a full series of turn maneuvers can be fullycompensated without interrupting reconnaissance.

Thus, a significant teaching of the present invention is an opticalassembly which is able to maintain a given pointing angle in inertialspace (or to maintain pointing at a given target position). Bydecoupling the reconnaissance mission from the airframe movements, thisteaching provides immense advantages in airborne reconnaissance.

Some of the innovative teachings in the present application could beused in imaging systems other than airborne reconnaissance system,particularly where the available platform may have significant propermotions. However, the primary application contemplated is aerialreconnaissance, and the innovations taught herein are particularlyadvantageous in that context.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described with reference to theaccompanying drawings, wherein:

FIG. 1A is a sectional view, from the left (port) side of the aircraftof key portions of the optical train of the presently preferredembodiment.

FIG. 1B schematically shows a sectional view of the optical train in thepresently preferred embodiment, as seen from the rear of the aircraft.The top portion of FIG. 1B also shows, in elevation, the rotation of theimager 110.

FIGS. 2A and 2B are schematic diagrams of the signal processingpreferably used on the outputs of the imager 110.

FIGS. 3A and 3B schematically show how the rotation of the imager 110compensates for the apparent image shift caused by rotation of therotating prism 220.

FIG. 4 schematically shows the control logic used to control themovements of the imager.

FIGS. 5A and 5B show the relative locations and mechanical connectionsof key portions of the mechanisms which move the rotating prism and theimager.

FIGS. 6A through 6C show the presently preferred embodiment of theimager package.

FIG. 7 shows the lens system preferably used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment, wherein these teachings are advantageously applied to theparticular problems of medium-altitude reconnaissance. However, itshould be understood that this embodiment is only one example of themany advantageous uses of the innovative teachings herein.

General System Configuration

FIG. 1A shows key portions of the presently preferred embodiment. A lensassembly 200 images a track on the ground, as seen through externalwindow 112, onto imager 110. The optical train is coupled through afixed prism 210 and a rotating prism 220. The rotating prism 220 ismounted so that it can pivot along the roll axis of the aircraft, and,as shown in FIG. 1B, can direct the field of view 111 from a positionnear the port horizon of the aircraft to a position near the starboardhorizon of the aircraft.

FIG. 1A is a sectional view, from the left (port) side of the aircraft,of key portions of the optical train of the presently preferredembodiment.

FIG. 1B schematically shows a sectional view of the optical train in thepresently preferred embodiment, as seen from the rear of the aircraft.The top portion of FIG. 1B also shows, in elevation, the rotation of theimager 110.

FIGS. 3A and 3B schematically show how the rotation of the imager 110compensates for the apparent image shift caused by rotation of therotating prism 220. FIG. 3A shows the nadir position, and FIG. 3B showsthe horizon position. As this pair of figures shows, a 90° rotation ofthe prism 220 is compensated by an equal rotation of the imager 110.

Another way to think of this operation is that the imager 110 is imagedonto a line which continuously sweeps along the ground in the sametrack. In order to maintain this optical alignment, one of thecomponents of motion of the imager 110 is a rotational component,synchronized with the rotation of a prism 220. Another component of therotational drive of imager 110 is synchronized to the airframe yaw axiscomponent, as derived from gyro information. A further static componentmay be added in for crab compensation.

Of course, other means well known to those skilled in the art could beused instead to compensate for apparent image rotation caused by therotation of prism 220. However, a particular advantage of performingthis compensation using rotation of the imager 110 is that the samemotion of imager 110 is also used to provide compensation for yaw axisattitude corrections.

One significant class of alternative embodiments permits both of theprisms 210 and 220 to be rotated. Rotation of the "fixed" prism 210permits the field of view to be moved in the fore-aft direction. Thus,this additional motion can be used for pitch compensation. Rotation ofthe prism 210, like rotation of prism 220, produces apparent imagerotation which must be compensated. Thus, in-plane rotation of theimager is preferably driven by a control signal which includes twocomponents corresponding to the rotations of the two prisms. Systemswhich include the capability for rotation of prism 210 should includethe capability for in-plane rotation of the imager, but may or may notalso include the capability for in-plane translation of the imager.Depending on the optics and window geometry used, rotation of prism 210may permit a much larger range of pointing angles in the pitch axis thanis permitted by in-plane translation of the imager.

Lens Assembly

FIG. 7 shows the presently preferred embodiment of the lens assembly200. (This drawing is seen along the axis of the optical train, so thatthe prisms are shown simply as transmissive elements.) The lens assembly200 is generally conventional. However, it does have one distinctivefeature which should be noted: it has an external entrance pupil,positioned between the rotatable prism 220 and the stationary prism 210.This means that the apertures of the two prisms can be made relativelysmall, for a given field of view. This has two advantages. First, therotating mass of the rotatable prism can be small, so that the netbandwidth (of the electro-mechanical system defined by the roll attitudesensors and the corresponding response of the rotating prism) will belarge, which is desirable. Second, the smaller the physical size of therotating prism, the smaller the external window 112 can be made (for agiven lateral field of view), which is also desirable.

In some applications, it may also be advantageous to include filteringto reduce blue and near-UV wavelengths, to reduce haze. The CCDsensitivity curve of a silicon device will inherently have a slightroll-off in the blue, which is advantageous.

Imager

Preferably the imager 110 is configured as two linear charged coupleddevices (CCDs), mechanically abutted. In the presently preferredembodiment, each of the linear CCDs has 6000 pixels, so that the imageis 12,000 pixels wide. Linear CCDs are generally well known and widelyavailable. However, the presently preferred embodiment of the imagerwill now be described in detail, for clarity and because some of thefeatures of this imager are particularly advantageous in the context ofthe system described.

In the presently preferred embodiment, each of the linear CCDs has 6000active photosite elements and 20 dark reference elements. Two transfergates provide parallel transfer: one transfer gate transfers the chargefrom each of the odd-numbered photosites to a site in a CCD shiftregister, and the other transfer gate transfers the charge from theeven-numbered photosites to another CCD shift register. Each of the twoshift registers can be clocked to transfer charge packets along itslength to a charge detector and output amplifier. Thus, there are atotal of four output lines from the two CCD chips.

The dark reference elements (as is well known in the CCD art) permit thedark current to be subtracted from the raw output, to get a bettermeasure of the optical signal. (A CCD photosite will collect a certainamount of charge at zero illumination, due to traps and other thermallysensitive effects. This amount of charge is referred to as "darkcurrent.") The transfer gates access these dark reference elementsanalogously to the active photosites.

To facilitate butting the two CCDs together, a trench is etched at thebutt end during device processing. The sidewalls of this trench arepassivated with channel stop doping and field oxide. This means that asawing operation can cut through the trench bottom, with reduced risk ofdestroying the last photosite. This structure also reduces chargeleakage into the last photosites. Other known methods are also used toavoid spurious signals: for example, portions of the second metal levelare used to screen areas other than active photosites from illumination,and a guard ring preferably surrounds active areas.

FIG. 6A shows the presently preferred embodiment of the package used forthe CCD imager chips. A thick polycrystalline silicon substrate 602 hasa thick film insulating glaze 604 and a screen-printed thick-filmmetallization 606 (preferably gold) overlaid on it. The conductor 606 ispatterned to bring leads (for signals, power, ground, etc.) outside ofthe hermetic seal. Another thick film insulating glaze layer 608overlies conductor layer 606 in the seal area, to provide a planarsealing surface.

The window 630 (which is preferably sapphire, but may be quartz or othermaterial) is given a thin patterned metal coat 616 on its backside inthe seal area. In the presently preferred embodiment, this is a thinlayer of Cr/Ni/Au, but other materials may be used instead. This may bedeposited, e.g., by evaporation or sputtering.

A silicon frame 620 forms a connection from window 630 to substrate 602.The actual CCD chips are epoxied to substrate 602 inside the ringdefined by silicon frame 620. (Preferably this epoxy attachment isperformed under a microscope, at a workstation with micrometermanipulation, so that the relative alignment of the CCD chips can beprecisely defined.) Stitch bonding is used to connect bond pads on theCCD chips to the traces of metallization 606. A thick film metallization610 (preferably palladium/silver) is applied to both sides of the frame620, to permit formation of a solder bond. (The same metallization ispreferably applied over glaze 608.) The frame 620 is then soldered(joint 612) to window 630, and this joint is tested for hermeticity.Frame 620 and window 630 can then be soldered (joint 614) to the metalring on substrate 602, enclosing the CCD chips within a hermetic seal.The package is then purged and backfilled (e.g., with dry helium), andtip-off hole 632 (which was preferably previously metallized) is sealed.

As FIG. 6B shows, the imager thus packaged is preferably attached bystud/nut assemblies 658 to a rigid mount 650 (which includesprecision-machined bosses, to assure accurate location of the imager 110parallel to the focal plane). This rigid mount 650 is connected to themechanical elements described below, to translate the imager as desired.A thermoelectric cooler 652 is spring-loaded (by springs 654) to makegood contact with the substrate 602 (assisted by thermal grease 652).

FIG. 6C shows how the connections to stud/nut assemblies 658 allow forthermal expansion. A tight hole 682 (in substrate 602) is a precisionfit. A slot 684 permits free movement in one dimension only. Loose hole686 permits free movement in two dimensions, but does restrictout-of-plane movement.

A linear CCD can have great advantages over use of an area CCD in aerialphotography. However, various of the inventive concepts set forth hereincould also be applied (less preferably) to systems using area imagerCCDs. Various of the inventive concepts taught by the presentapplication could also be applied to systems using quasi-linear CCDs,such as time delay and integrate (TDI) devices.

It should also be recognized that the "linear" imager used does notstrictly have to have a by-1 configuration. For example, a CCD with twoor three parallel lines of sensing sites could also be used, and mighteven be preferable for some purposes (e.g. color imaging, or to provideimmunity to single-pixel defects). For another example, it would also bepossible to use optical combinations of more CCDs than the two used inthe presently preferred embodiment. It should also be recognized thatimagers using other electro-optic technologies (such as photodiodes,charge-imaging matrix technology, electron multipliers, etc.) could alsobe used. The innovations taught herein can also be extended to systemsusing wavelengths beyond the visible and near-infrared range used by thepresently preferred embodiment.

The CCD imager is preferably temperature-stabilized. In the presentlypreferred embodiment, the paired CCDs are mounted on a silicon substrate(which provides an excellent thermal match). A sapphire cover is used toprovide a hermetically sealed front window. (Sapphire has a good thermalmatch, but of course other materials could be used instead.) The siliconsubstrate is preferably mounted on a thermoelectric cooler, whichmaintains a mean temperature of 10° C.

Mechanical Implementation

FIGS. 5A and 5B show the relative locations and mechanical connectionsof key portions of the mechanism which moves the rotating prism, themechanisms which move the imager, the lens assembly, and the rotatingand fixed prisms.

The lens assembly subhousing 106 is preferably housed within an outerhousing 502. This prevents any stress on the optical elements, and alsoprovides a convenient subassembly. The outer housing 502 also supports asubhousing 503, which supports the rotating prism 220 and the motor (andtachometer) assembly 504 which drives prism 220. The fixed prism 210 ispreferably supported by the lens assembly subhousing 106.

Two large bearings 512 support the focal plane assembly. In thepresently preferred embodiment these bearings are about 10 inchesacross, and the maximum width of the module is about 15 inches. A motor514 (extending out from the outer housing) drives the rotation of theselarge bearings, using a simple spur gear and bull gear assembly. (Asdiscussed above, this motion is controlled in accordance with both rolland yaw signals (including crab correction)).

A linear translation mechanism is supported on rotating bearings 512.(The linear translation mechanism is seen most clearly in FIG. 5B, whichis a plan view at section A--A of FIG. 5A.)

One axis of linear translation is available in the focal plane. Thistranslation is driven by a motor assembly 1004. The motor assembly 1004drives a ball screw 1002, which is connected to a support frame 1009.The support frame 1009, translated by motor assembly 1004, is supportedat one side by ball screw 1002. On the other side it is shown assupported by a linear bearing assembly 1011. (However, in the latestmodification to the presently preferred embodiment, a linear slide isused instead.) In the presently preferred embodiment, the availabletotal motion of the in-plane translation mechanism is about 2.5 inches.(For comparison, the width of the optically active area of the buttedCCDs is about 4.75 inches.)

A shaft encoder assembly 1008 is located at the other end of the shaftwhich extends through ball screw 1002, at the end opposite from themotor assembly 1004. (This provides compact physical dimensions.)

The support frame 1009 supports the range focus assembly 520. The rangefocus assembly, within the support frame translated by a motor assembly1004, provides the range focus movement of the imager. In the presentlypreferred embodiment, the range focus motion is accomplished by a camand cam follower assembly. The available total motion is about 0.5 inch(although this is more than is strictly necessary).

FIG. 5B also shows the physical location of the electronics 530 whichare used, in the presently preferred embodiment, to perform thefunctions shown in FIGS. 2 and 4.

In the presently preferred embodiment, the range focus mechanism iscarried by the mechanism which effects pitch axis movement (in-planetranslation), and that mechanism in turn is carried by the mechanismwhich effects yaw axis movement (rotation). Alternative sequences of"nesting" of the available movements could be used. However, onesignificant teaching of the present application is that the mechanismwhich effects rotation should preferably carry (directly or indirectly)the mechanism which effects in-plane translation, rather than viceversa. (In the presently preferred embodiment, the range focus mechanismis carried by the translation motion, but alternatively the range focusmechanism could have been constructed to carry the translation motionmechanism, or the translation and rotation mechanisms.)

Of course, the fact that multi-axis motion compensation is performed bymotion of the imager in the focal plane does not mean that additionalmovement of the lens system, or even of the whole assembly, could notalso be used if desired. For example, it would obviously be possible tomount the whole assembly retractably, so that it could selectably bemoved back to a more protected position inside the air vehicle whenreconnaissance was not possible. Such an embodiment, where the lensassembly is not moved during operation for motion compensation, isconsidered to fall within references to a "substantially fixed" lensassembly, as used in the specification and claims of this application.

A further extension of the innovative ideas presented herein is that asystem, including multiple axis motion compensation by motion of theimager in the focal plane, could be combined with one or more availablemovements in the lens assembly or in other parts of the optical plane.That is, in the presently preferred embodiment, the imager has threeavailable components of motion: vertical translation, rotation, andhorizontal (in-plane) translation. It would be possible to embody someof these movements equivalently in other parts of the optical system.For example, the range/focus adjustment could optionally (and lesspreferably) be implemented as a motion of the lens assembly, or of somebut not all elements in the lens assembly. (This implementation isdefinitely less preferred, and would of course be subject to constraintson the entrance pupil at the rotating prism, so that focusing motionswould not degrade field of view, or cause darkening towards the edge ofthe field.

In another alternative embodiment, it would also be possible (althoughless preferable) to use moving prisms or mirrors near a pupil of anoptical train to compensate for an additional component of motionbesides roll axis attitude. For example, as discussed above, tworotating prisms could be used.

Similarly, it should be noted that it is not strictly necessary for theaxis of rotation of the rotatable element to be parallel to the rollaxis of the air vehicle. (If these axes are not parallel, simpletrigonometric transformations can correct the movements of the focalplane to allow for the pitch and yaw axis components of the motion ofthe rotatable element.) Again, this implementation would be lesspreferable, but such implementations can make use of some of theinnovative ideas presented.

Control

In the presently preferred embodiment, the control system (whichcontrols the movements of the imager) is implemented as an analogsystem.

The crab input is provided as an external input, and can come, e.g.,from the navigation equipment.

If a pitch correction is made while the imager has been rotated (i.e.while the imager is not parallel to the pitch axis), then atrigonometric correction factor (e.g. the cosecant of the angle by whichthe imager position is away from the pitch axis) is preferablymultiplied into the imager shift amount. Similarly, if a rotation mustbe performed (for yaw correction or for prism rotation compensation)while the imager is off center (e.g. due to pitch correction), it may bedesirable to perform a pixel shift operation to maintain accuratereproduction of straight lines in-track. At low magnitudes, some ofthese errors can simply be tolerated.

The control system has two modes: a position-determining mode and arate-sensing mode. In FIG. 4, these modes are shown as standby (STBY)and operate (OPR) modes of three switches 402. The imager is translatedto a desired position (in the first mode), and then the system isswitched over into the stabilization mode. If the imager's positionbecomes out of bounds, it is translated back to a central position, theposition values are reset if needed, and active stabilization isresumed. Ideally the resets will not come very often (e.g., everyfifteen minutes or so).

Roll, pitch, and yaw rate inputs, from rate gyros, are shown as inputvalues R, P, and Y.

The roll mirror position and aircraft roll angle are applied as inputsto a computation 404 (preferably implemented in a microprocessor), whichcalculates the instantaneous depression angle θ_(D). This angle is theangle by which the center of the field of view is below the horizon.This angle is used to define the trigonometric transformations whichrelate motions of the imager to motions of the field of view. The pitchrate input P is transformed to Psin θ_(D) +Ycos θ_(D). The yaw rateinput Y is transformed to Ysin θ_(D) +Pcos θ_(D).

Three electromechanical control loops are used to govern the movementsof the field of view. Loop 410 provides an output 442 to control motor1004 (shown in FIG. 5B) to govern the in-plane translation of the imager110. Loop 420 provides an output 444 to control motor 514 (shown in FIG.5A) to govern the rotation of the imager 110. Loop 430 provides anoutput 446 to control motor 504 (shown in FIG. 5A) to govern thepointing of the rotatable prism 220.

Each of the loops 410, 420, and 430 preferably has two modes, selectedby a switch 402. In the "STBY" mode position transducers 412, 422, and432 are used in the loops 410, 420, and 430 (respectively). In the "OPR"mode velocity transducers 411, 421, and 431 are used to provide thefeedback sources in the loops 410, 420, and 430 (respectively). Theamplification and feedback arrangements of these control loops areconventional.

Note that the transformed yaw rate input (Ysin θ_(D) +Pcos θ_(D)) iscombined with the roll rate input R to define the input to loop 420,which governs focal plane rotation.

Positioning commands can be applied to the loops in their "STBY" modes.For example, a roll pointing command can be applied to loop 430, in the"STBY" modes, to change the mirror pointing to select a field-of-view.(For example, a pilot may wish to image a particular area which islaterally displaced from the track beneath the airplane.) Similarly,quasi-static yaw axis inputs (to correct for crab angle, i.e. driftangle due to crosswind, with an appropriate trigonometric correction,i.e. multiplication by sin θ_(D)) can be applied to loop 420 in its STBYmode. (Note that the three loops shown need not switch modes together.)

The range focus movement is preferably not included in the controlsystem shown. Real-time control of range focus is not normally critical,and may be performed directly by the pilot.

Signal Processing

The imager output is preferably processed in a number of ways. The rawoutput from the CCD wells will not only contain electrical variationswhich correspond to the detailed appearance of the scene, but will alsoinclude variations due to several other sources. These other sourcesinclude overall changes in illumination; changes in average brightnesslevel of the objects in the scene; haze or clouds between the platformand the scene; and charge variations due to electrical noise in the CCD.

FIG. 2 shows generally the routing preferably used. The two CCD chips inimager 110 each have separate outputs for odd and even pixels, so thatfour parallel channels are used at first. For example, four separatebuffer amplifiers 901 and prescalers 902 are shown.

Prescaling

In the presently preferred embodiment, 9-bit quantization is used withprescaling. Prescalers 902 are preferably configured as conventionalR/2R ladders, controlled by the signal fed back from later stages.

However, it is contemplated that 10-bit quantization, with fixed gain,may ultimately be preferred. Output signals from the CCD preferably usedwill correspond to the range from about 600,000 electrons per wellsaturated signal, down to about 360 electrons per well residual noisesignal. Ten bits of resolution, at a fixed scale, can providesubstantially adequate measurement over this range.

Analog filtering

Before the CCD output is digitized, it is filtered in the analog domainto remove low-frequency noise. This filtering operation is done with atransversal filter 904 which embodies essentially the same transferfunction as a correlated double sampler. A following negative clippingstage 906 keeps the signal in bounds. The four 6 MHz data streams arethen multiplexed down to two 12 MHz streams. (The signal format used issuch that this combining step can be performed by an analog adder 908.)

Note that the filter 904 performs a function which is different from theanalog preliminary stages normally used in any digital system (e.g.offset correction, preamplification, prescaling, and/or anti-aliasingfilter). This transversal filter removes the low-frequency noisecomponents, including kTC and 1/f noise components.

Conventionally, in an analog front end system, such filtering will beaccomplished by correlated double sampling. In fact, the transversalfilter function preferably used has the same transfer function as acorrelated double sampler.

However, one important advantage of the transversal filter is that it isimpossible to do matched filtering after correlated double sampling. Apossible deterrent to using the transversal filter is that correlateddouble sampling also strips the pixel clock feedthrough, which theanalog transversal filter does not. However, stripping the pixel clockfeedthrough is not necessary if the video signal is to be immediatelydigitized, as it is in the presently preferred application.

Image Brightness Compensation

A/D converters 912 provide 9-bit values for each of the two signalstreams, and multiplexer 914 combines the data into a single 9-bit dataflow at 24 MHz.

The CCD outputs will be affected by pixel nonuniformities ("pixelsignatures") and by position-dependent brightness variation. Pixelsignatures result from the nonuniform areas and capacities of theindividual collection sites. Position-dependent brightness variationresults from the brightness fall-off of the lens: as the imager is movedaway from the center of the focal plane, the brightness of the imagewill be reduced.

In the presently preferred embodiment, image brightness compensation isaccomplished (in gain correction stage 916) by multiplying the digitizedvalue of each pixel by a scaling factors. The scaling factors are storedin a PROM 918, as one value for each pixel of the imager. These scalingfactors compensate both for the different sensitivities of the variouspixels, and also for position-dependent variation in image brightness.(As discussed above, the lens has some brightness fall-off near the edgeof its field.)

Gain Control

For optimal recognition, it is desirable to adjust the scaling andoffset of the output so that the detail information is clearlyrecognizable. This is conventionally accomplished by an automatic gaincontrol (AGC) circuit of some sort. A significant difficulty in theprior art has been to perform detail enhancement without introducingartifacts into the image.

The presently preferred embodiment uses a two-dimensional "histogrammer"approach to emphasize the detail information in the scene. Long-termaverage minimum and maximum values are separately tracked (byhistogrammer 920), based on preceding pixels in-track and on all pixelsin the cross-track direction. Stages 922 and 924 then scale the pixelvalues to these two separately-tracked values. Haze subtract stage 922removes the average minimum, and "AGC" stage 924 scales the pixel valueswith respect to the average maximum. (Note that these are controlled byinputs from the histogrammer stage 920.)

In addition to the filtering introduced by the histogrammer approach,manual switching (in the analog domain) is used (in the presentlypreferred embodiment) to remove "cloud spikes" (i.e. spurious horizontallines caused by atmospheric variations between the platform and theobject being imaged). In the presently preferred embodiment, a pilot oroperator would directly input a value indicating his estimate of thecloud brightness level seen by the imager, and this value defines thecloud clipping level. However, alternatively, an additional automaticcontrol loop could be used instead. Control subsystems which willprovide automatic compensation for these factors are generally familiarto those skilled in the art. This operation is shown as box 910 in FIG.2.

Adaptive Time Constant

In the presently preferred embodiment, the time constants for both theminimum (i.e., haze-subtract) and maximum (i.e., AGC) level tracking arereduced by an order of magnitude when a step change in scenereflectivity is detected. An overflow/underflow event counter is used tomonitor the number of overflow or underflow events seen by thecomparators which come after the AGC range scaler networks. Underreasonably normal scaling conditions, only a moderate level of overflowand/or underflow events will be seen. However, a step change in scenereflectivity will cause a sudden large increase in the number ofoverflow or underflow events. When the counter detects that the rate ofsuch events has increased above a certain level, it will trigger achange in the time constants associated with formation of the minimumand maximum levels. This has the advantage that frame blackout or framewhiteout resulting from a sharp change in the image is avoided.

In the presently preferred embodiment, the time constant change isaccomplished by replicating data values being loaded into a register.That is, to reduce the time constant by a factor of ten, each line'sminimum and maximum values are loaded ten times into the averagingoperation, rather than only once.

Values for missing pixels (at the butt between the two CCD chips) aregenerated by averaging the values from the adjacent "live" pixels.

Image Rectification

"Image rectification" is the process of removing the component ofdistortion which is caused by unequal in-track and cross-track groundsample distances. The purpose of image rectification is to ensure thateach pixel corresponds to an area on the ground which has approximatelyequal dimensions in the in-track and cross-track directions.

Image rectification is accomplished (in logic not shown in FIG. 2) bygeneration of a video line rate governed by the following relationship:

    R=(F*V* sin θ)/(N*P*H),

where R=the video line rate

F=sensor focal length

P=pitch of CCD pixel

N=implied pixel grouping integer

V=vehicle ground speed

H=vehicle altitude

θ=the depression angle. (The sin θ term compensates for projection ontoa focal plane which is not normal to the ground.)

The implied pixel grouping integer N is changed as needed to avoidinfringing the system data rate and/or maximum sensor line rate limits.

Bandwidth Limiting

The implied pixel grouping integer N is determined by the system datarate and/or maximum sensor line rate. In the presently preferredembodiment, the maximum sensor line rate is 2000 lines per second. Aslong as neither of these factors is limiting, the grouping integer N isleft equal to 1 (i.e. the line rate is not reduced). However, when oneof these limits is reached (for example, when V/H increases duringflight), N is increased to a higher integer. This means that pixelgrouping takes place, so that the line rate is halved. Preferably thenumber of pixels per line is also halved. (This means that the net datarate is being reduced by N².)

Alternatively, the parameters for line and pixel grouping may bedecoupled. This would mean that retranslation of the output still wouldbe relatively simple (since image pixels would be combined intorectangular blocks), but less drastic steps in data rates would beavailable. The pixel grouping integer N is set as an input to an I/Omultiplexer, which accomplishes pixel grouping.

The rules defining the pixel grouping imager have hysteresis built in.That is, the break points used to define pixel grouping are differentunder increasing V/H conditions and decreasing V/H conditions. Thishelps to avoid line rate jitter.

Data Output

In the presently preferred embodiment, the reconnaissance system isdesigned to be borne by an airplane, and the image data output is savedon a conventional multitrack digital magnetic tape recorder. However, inalternative embodiments an RF downlink (real-time or buffered) could beused instead. This might be particularly advantageous where differentplatforms (such as drones) are used for the reconnaissance mission.

As will be recognized by those skilled in the art, the innovativeconcepts described herein can be modified and varied over a tremendousrange of applications, and accordingly their scope is not limited exceptby the claims.

What is claimed is:
 1. A reconnaissance system, for mounting in an airvehicle, comprising:an optical train,positioned to define a fixed focalplane and comprising at least one reflector rotatable to change theexternal solid angle imaged onto said focal plane; sensors connected tosense multiple parameters corresponding to attitude components of saidair vehicle; an electro-optic imager, movable within the focal planedefined by said lens; and a controller, actuator and linkage which areconnected to move said imager within said focal planein response tooutputs of said sensors, to compensate for motions of the air vehicle inat least one rotational axis, and also to compensate for imageorientation changes due to rotation of said rotatable reflector, whereinsaid rotatable reflector is mounted to rotate about an axis which issubstantially parallel to the roll axis of the air vehicle, and whereinsaid imager is moved in accordance with pitch axis motions of the airvehicle.
 2. A reconnaissance system, for mounting in an air vehicle,comprising:an optical train,positioned to define a fixed focal plane andcomprising at least one reflector rotatable to change the external solidangle imaged onto said focal plane; sensors connected to sense multipleparameters corresponding to attitude components of said air vehicle; anelectro-optic imager, movable within the focal plane defined by saidlens; and a controller, actuator and linkage which are connected to movesaid imager within said focal planein response to outputs of saidsensors, to compensate for motions of the air vehicle in at least onerotational axis, and also to compensate for image orientation changesdue to rotation of said rotatable reflector, wherein said rotatablereflector is mounted to rotate about an axis which is substantiallyparallel to the roll axis of the air vehicle, and wherein saidcontroller is connected to cause rotation of said imager, during atleast some time periods, at a rate which is equal to the sum ofthe imagerotation rate due to rotation of said reflector plus the rate ofrotation of said air vehicle about the yaw axis thereof.
 3. Areconnaissance system, for mounting in an air vehicle, comprising:anoptical train,positioned to define a fixed focal plane and comprising atleast one reflector rotatable to change the external solid angle imagedonto said focal plane; sensors connected to sense multiple parameterscorresponding to attitude components of said air vehicle; anelectro-optic imager, movable within the focal plane defined by saidlens; and a controller, actuator and linkage which are connected to movesaid imager within said focal planein response to outputs of saidsensors, to compensate for motions of the air vehicle in at least onerotational axis. also to compensate for image orientation changes due torotation of said rotatable reflector, wherein said actuator and linkageare connected to move said imager within said focal plane with twodegrees of freedom, including rotation and one direction of translation.4. A reconnaissance system, for mounting in an air vehicle,comprising:an optical train,positioned to define a fixed focal plane andcomprising at least one reflector rotatable to change the external solidangle imaged onto said focal plane; sensors connected to sense multipleparameters corresponding to attitude components of said air vehicle; anelectro-optic imager movable within the focal plane defined by saidlens; and a controller, actuator and linkage which are connected to movesaid imager within said focal planein response to outputs of saidsensors, to compensate for motions of the air vehicle in at least onerotational axis, and also to compensate for image orientation changesdue to rotation of said rotatable reflector, wherein said controller,actuator and linkage are connected to move said imager within said focalplane to compensate for yaw and crab of the air vehicle.
 5. Amedium-altitude reconnaissance system, comprising:a lens system,mountable to an air vehicle at a substantially fixed pointing angle; animager movable with multiple degrees of freedom, and aligned with saidlens assembly in an optical train which images said imager onto anexternal strip; a movable pointing element, aligned with said lenssystem to steer the field of view thereof; control electronics,connected to control movements of said imager at least partially incompensation for movements of the air vehicle to which said lensassembly is mounted; and wherein said lens assembly is not movable tocompensate for movements of the air vehicle, wherein said controlelectronics are connected to control movements of said imager inaccordance with parameters including roll, pitch, and yaw rates of saidair vehicle.
 6. A medium-altitude reconnaissance system, comprising:alens system, mountable to an air vehicle at a substantially fixedpointing angle; an imager movable with multiple degrees of freedom, andaligned with said lens assembly in an optical train which images saidimager onto an external strip; a movable pointing element, aligned withsaid lens system to steer the field of view thereof; controlelectronics, connected to control movements of said imager at leastpartially in compensation for movements of the air vehicle to which saidlens assembly is mounted; and wherein said lens assembly is not movableto compensate for movements of the air vehicle, wherein movement of saidimager within said focal plane is controlled to compensate for motionsof the air vehicle about the yaw axis thereof.
 7. A medium-altitudereconnaissance system, comprising:a lens system, mountable to an airvehicle at a substantially fixed pointing angle; an imager movable withmultiple degrees of freedom, and aligned with said lens assembly in anoptical train which images said imager onto an external strip; a movablepointing element, aligned with said lens system to steer the field ofview thereof; control electronics, connected to control movements ofsaid imager at least partially in compensation for movements of the airvehicle to which said lens assembly is mounted; and wherein said lensassembly is not movable to compensate for movements of the air vehicle,wherein said imager is movable, within the focal plane defined by saidoptical train, with at least two degrees of freedom.
 8. Amedium-altitude reconnaissance system, comprising:a lens system,mountable to an air vehicle at a substantially fixed pointing angle; animager movable with multiple degrees of freedom, and aligned with saidlens assembly in an optical train which images said imager onto anexternal strip; a movable pointing element, aligned with said lenssystem to steer the field of view thereof; control electronics,connected to control movements of said imager at least partially incompensation for movements of the air vehicle to which said lensassembly is mounted; an wherein said lens assembly is not movable tocompensate for movements of the air vehicle, wherein said imager ismovable, within the focal plane defined by said optical train, withexactly two degrees of freedom.
 9. A medium-altitude reconnaissancesystem, comprising:a lens system, mountable to an air vehicle at asubstantially fixed pointing angle; an imager movable with multipledegrees of freedom, and aligned with said lens assembly in an opticaltrain which images said imager onto an external strip; a movablepointing element, aligned with said lens system to steer the field ofview thereof; control electronics, connected to control movements ofsaid imager at least partially in compensation for movements of the airvehicle to which said lens assembly is mounted; and wherein said lensassembly is not movable to compensate for movements of the air vehicle,wherein said imager is movable, within the focal plane defined by saidoptical train, in rotation and in one direction of translation
 10. Amedium-altitude reconnaissance system, comprising:a lens system,mountable to an air vehicle at a substantially fixed pointing angle; animager movable with multiple degrees of freedom, and aligned with saidlens assembly in an optical train which images said imager onto anexternal strip; a movable pointing element, aligned with said lenssystem to steer the field of view thereof; control electronics,connected to control movements of said imager at least partially incompensation for movements of the air vehicle to which said lensassembly is mounted; and wherein said lens assembly is not movable tocompensate for movements of the air vehicle, wherein said controller,actuator and linkage are connected to move said imager within said focalplane to compensate for yaw and crab of the air vehicle.
 11. Amedium-altitude reconnaissance system, comprising:a lens system,mountable to an air vehicle at a substantially fixed pointing angle; animager movable with multiple degrees of freedom, and aligned with saidlens assembly in an optical train which images said imager onto anexternal strip; a movable pointing element, aligned with said lenssystem to steer the field of view thereof; control electronics,connected to control movements of said imager at least partially incompensation for movements of the air vehicle to which said lensassembly is mounted; and wherein said lens assembly is not movable tocompensate for movements of the air vehicle, wherein said controller,actuator and linkage are also connected to move said imager normal tosaid focal plane to achieve a desired range focus.
 12. A medium-altitudereconnaissance system comprising:a lens system, mountable to an airvehicle at a substantially fixed pointing angle; an imager movable withmultiple degrees of freedom, and aligned with said lens assembly in anoptical train which images said imager onto an external strip; arotatable optical element which, with said lens system and said imager,forms an optical train which images external objects onto said imager,said rotatable optical element being positioned to steer the field ofview of said optical train; wherein said rotatable optical element islocated in very close proximity to a pupil of said lens system, whereinmotion of said imager within said focal plane is controlled tocompensate for motions of the air vehicle about the yaw axis thereof.13. A medium-altitude reconnaissance system, comprising:a lens system,mountable to an air vehicle at a substantially fixed pointing angle; animager movable with multiple degrees of freedom, and aligned with saidlens assembly in an optical train which images said imager onto anexternal strip; a rotatable optical element which, with said lens systemand said imager, forms an optical train which images external objectsonto said imager, said rotatable optical element being positioned tosteer the field of view of said optical train; wherein said rotatableoptical element is located in very close proximity to a pupil of saidlens system, wherein said imager is movable within said focal plane withtwo degrees of freedom, including rotation and one direction oftranslation.
 14. A method for performing aerial reconnaissance,comprising the steps of:flying an air vehicle along a track which iswithin view of a desired area to be imaged, wherein said air vehicleincludes:a lens system mounted substantially fixedly; an electro-opticimager mounted in the focal plane of said lens system, wherein saidimager is controllably movable in at least one direction of translationsubstantially within the focal plane of said lens assembly, a rotatableoptical element which, with said lens system and said imager, forms anoptical train which images external objects onto said imager, saidrotatable optical element being positioned to steer the field of view ofsaid optical train; imaging the desired area, while maneuvering the airvehicle freely; and moving said imager during said imaging step, tocompensate at least partially for motions of the air vehicle, whereinsaid imager is moved during imaging in accordance with parametersincluding roll, pitch, and yaw rates of the air vehicle.
 15. A methodfor performing aerial reconnaissance, comprising the steps of:flying anair vehicle along a track which is within view of a desired area to beimaged, wherein said air vehicle includes:a lens system mountedsubstantially fixedly; an electro-optic imager mounted in the focalplane of said lens system, wherein said imager is controllably movablein at least one direction of translation substantially within the focalplane of said lens assembly, a rotatable optical element which, withsaid lens system and said imager, forms an optical train which imagesexternal objects onto said imager, said rotatable optical element beingpositioned to steer the field of view of said optical train; imaging thedesired area, while maneuvering the air vehicle freely; and moving saidimager during said imaging step, to compensate at least partially formotions of the air vehicle, wherein said rotatable element is mounted torotate about an axis which is substantially parallel to the roll axis ofthe air vehicle, and wherein said imager is translated in accordancewith pitch axis motions of the air vehicle.
 16. A method for performingaerial reconnaissance, comprising the steps of:flying an air vehiclealong a track which is within view of a desired area to be imaged,wherein said air vehicle includes:a lens system mounted substantiallyfixedly; an electro-optic imager mounted in the focal plane of said lenssystem, wherein said imager is controllably movable in at least onedirection of translation substantially within the focal plane of saidlens assembly, a rotatable optical element which, with said lens systemand said imager, forms an optical train which images external objectsonto said imager, said rotatable optical element being positioned tosteer the field of view of said optical train; imaging the desired area,while maneuvering the air vehicle freely; and moving said imager duringsaid imaging step, to compensate at least partially for motions of theair vehicle, wherein motions of said rotatable element and of saidimager within said focal plane provide compensation for motions of theair vehicle about both roll and pitch axes thereof.
 17. A method forperforming aerial reconnaissance, comprising the steps of:flying an airvehicle along a track which is within view of a desired area to beimaged, wherein said air vehicle includes:a lens system mountedsubstantially fixedly; an electro-optic imager mounted in the focalplane of said lens system, wherein said imager is controllably movablein at least one direction of translation substantially within the focalplane of said lens assembly, a rotatable optical element which, withsaid lens system and said imager, forms an optical train which imagesexternal objects onto said imager, said rotatable optical element beingpositioned to steer the field of view of said optical train; imaging thedesired area, while maneuvering the air vehicle freely; and moving saidimager during said imaging step, to compensate at least partially formotions of the air vehicle, wherein motion of said imager within saidfocal plane is controlled to compensate for motions of the air vehicleabout the yaw axis thereof.
 18. A method for performing aerialreconnaissance, comprising the steps of:flying an air vehicle along atrack which is within view of a desired area to be imaged, wherein saidair vehicle includes:a lens system mounted substantially fixedly; anelectro-optic imager mounted in the focal plane of said lens system,wherein said imager is controllably movable in at least one direction oftranslation substantially within the focal plane of said lens assembly,a rotatable optical element which, with said lens system and saidimager, forms an optical train which images external objects onto saidimager, said rotatable optical element being positioned to steer thefield of view of said optical train; imaging the desired area, whilemaneuvering the air vehicle freely; and moving said imager during saidimaging step, to compensate at least partially for motions of the airvehicle, wherein said rotatable element is mounted to rotate about anaxis which is substantially parallel to the roll axis of the airvehicle, and wherein said controller is connected to cause rotation ofsaid imager, during at least some time periods, at a rate which is equalto the sum ofthe image rotation rate due to rotation of said rotatableelement plus the rate of rotation of said air vehicle about the yaw axisthereof.
 19. A method for performing aerial reconnaissance, comprisingthe steps of:flying air vehicle along a track which is within view of adesired area to be imaged, wherein said air vehicle includes:a lenssystem mounted substantially fixedly ; an electro-optic imager mountedin the focal plane of said lens system, wherein said imager iscontrollably movable in at least one direction of translationsubstantially within the focal plane of said lens assembly, a rotatableoptical element which, with said lens system and said imager, forms anoptical train which images external objects onto said imager saidrotatable optical element being positioned to steer the field of view ofsaid optical train; imaging the desired area, while maneuvering the airvehicle freely; and moving said imager during said imaging step, tocompensate at least partially for motions of the air vehicle, whereinsaid imager is movable within said focal plane with two degrees offreedom, including rotation and one direction of translation.
 20. Amethod for performing aerial reconnaissance, comprising the stepsof:flying an air vehicle along a track which is within view of a desiredarea to be imaged, wherein said air vehicle includes:a lens systemmounted substantially fixedly; an electro-optic imager mounted in thefocal plane of said lens system, wherein said imager is controllablymovable in at least one direction of translation substantially withinthe focal plane of said lens assembly, a rotatable optical elementwhich, with said lens system and said imager, forms an optical trainwhich images external objects onto said imager, said rotatable opticalelement being positioned to steer the field of view of said opticaltrain; imaging the desired area, while maneuvering the air vehiclefreely; and moving said imager during said imaging step, to compensateat least partially for motions of the air vehicle, wherein said imageris moved within said focal plane to compensate for yaw and crab of theair vehicle.
 21. A method for performing aerial reconnaissance,comprising the steps of:flying an air vehicle along a track which iswithin view of a desired area to be imaged, wherein said air vehicleincludes:a lens system mounted substantially fixedly; an electro-opticimager mounted in the focal plane of said lens system, wherein saidimager is controllably movable in at least one direction of translationsubstantially within the focal plane of said lens assembly, a rotatableoptical element which, with said lens system and said imager, forms anoptical train which images external objects onto said imager, saidrotatable optical element being positioned to steer the field of view ofsaid optical train; imaging the desired area, While maneuvering the airvehicle freely; and moving said imager during said imaging step, tocompensate at least partially for motions of the air vehicle, whereinsaid imager is also movable normal to said focal plane to achieve adesired range focus.
 22. A method for performing aerial reconnaissance,comprising the steps of:flying an air vehicle along a track which iswithin view of a desired area to be imaged; wherein said air vehicleincludes:a substantially fixed lens system mounted to image ground-levelobjects onto a substantially linear electro-optic imager movably mountedin the focal plane of said lens system, wherein said imager is asubstantially linear imager, and is controllably movable to movesubstantially within the focal plane of said lens assembly with at leasttwo degrees of freedom, and wherein said imager and said lens assemblyare aligned in an optical train to image a desired portion of a groundsurface external to the air vehicle onto said imager, and wherein saidoptical train includes at least one rotatable element such that theground region imager onto said imager can be changed by rotating saidrotatable element, apart from any changes in the attitude or position ofsaid air vehicle; imaging the desired area, while maneuvering the airvehicle freely; and moving both said imager and said rotatable elementduring said imaging step, to compensate for motions of the air vehicleabout both pitch and roll axes thereof, wherein said rotatable elementis mounted to rotate about an axis which is substantially parallel tothe roll axis of the air vehicle, and wherein said imager is translatedin accordance with pitch axis motions of the air vehicle.
 23. A methodfor performing aerial reconnaissance, comprising the steps of:flying anair vehicle along a track which is within view of a desired area to beimaged; wherein said air vehicle includes:a substantially fixed lenssystem mounted to image ground-level objects onto a substantially linearelectro-optic imager movably mounted in the focal plane of said lenssystem, wherein said imager is a substantially linear imager, and iscontrollably movable to move substantially within the focal plane ofsaid lens assembly with at least two degrees of freedom, and whereinsaid imager and said lens assembly are aligned in an optical train toimage a desired portion of a ground surface external to the air vehicleonto said imager, and wherein said optical train includes at least onerotatable element such that the ground region imager onto said imagercan be changed by rotating said rotatable element, apart from anychanges in the attitude or position of said air vehicle; imaging thedesired area, while maneuvering the air vehicle freely; and moving bothsaid imager and said rotatable element during said imaging step, tocompensate for motions of the air vehicle about both pitch and roll axesthereof, wherein motion of said imager within said focal plane iscontrolled to compensate for motions of the air vehicle about the yawaxis thereof.
 24. A method for performing aerial reconnaissance,comprising the steps of:flying an air vehicle along a track which iswithin view of a desired area to be imaged; wherein said air vehicleincludes:substantially fixed lens system mounted to image ground-levelobjects onto a substantially linear electro-optic imager movably mountedin the focal plane of said lens system, wherein said imager is asubstantially linear imager, and is controllably movable to movesubstantially within the focal plane of said lens assembly with at leasttwo degrees of freedom, and wherein said imager and said lens assemblyare aligned in an optical train to image a desired portion of a groundsurface external to the air vehicle onto said imager, and wherein saidoptical train includes at least one rotatable element such that theground region imager onto said imager can be changed by rotating saidrotatable element, apart from any changes in the attitude or position ofsaid air vehicle; imaging the desired area, while maneuvering the airvehicle freely; and moving both said imager and said rotatable elementduring said imaging step, to compensate for motions of the air vehicleabout both pitch and roll axes thereof, wherein said imager is movablewithin said focal plane with two degrees of freedom, including rotationand one direction of translation.
 25. A method for performing aerialreconnaissance, comprising the steps of:flying an air vehicle along atrack which is within view of a desired area to be imaged; wherein saidair vehicle includes:a substantially fixed lens system mounted to imageground-level objects onto a substantially linear electro-optic imagermovably mounted in the focal plane of said lens system, wherein saidimager is a substantially linear imager, and is controllably movable tomove substantially within the focal plane of said lens assembly with atleast two degrees of freedom, and wherein said imager and said lensassembly are aligned in an optical train to image a desired portion of aground surface external to the air vehicle onto said imager, and whereinsaid optical train includes at least one rotatable element such that theground region imager onto said imager can be changed by rotating saidrotatable element, apart from any changes in the attitude or position ofsaid air vehicle; imaging the desired area, while maneuvering the airvehicle freely; and moving both said imager and said rotatable elementduring said imaging step, to compensate for motions of the air vehicleabout both pitch and roll axes thereof, wherein said imager is alsomovable normal to said focal plane to achieve a desired range focus.